The present invention relates generally to methods of forming dielectric layers and the devices fabricated therewith. More specifically, the present invention relates to methods of forming dielectric layers having high dielectric constants by depositing oxidizable materials over a semiconductor substrate and oxidizing the materials to form amorphous oxides. The invention also pertains to devices formed employing such oxides.
Increasing the performance of integrated circuits (ICs), both with regard to more complex functionality and higher speeds, is a primary goal of efforts in advancing the semiconductor arts. One method that has been extensively employed to achieve this goal is scaling, that is decreasing area or size of individual device components that are used to form such integrated circuits. For example, the gate length of a typical MOS transistor has been reduced over the past several years from several microns to fractions of a micron and gate lengths of 0.1 micron or less will soon be desired. Such scaling efforts have also effected the size of capacitors used in a variety of ICs such as DRAMS and SRAMS (dynamic and static random access memories, respectively).
While such scaling efforts have resulted in the desired increases in performance, generally such size reductions also impact at least some characteristics of the devices so xe2x80x9cscaled.xe2x80x9d For example, reducing the gate length of a transistor generally reduces the transistor""s output and decreasing the size of a capacitor generally reduces the capacitance or amount of charge such a capacitor can store. Fortunately, changing other features of devices such as transistors and capacitors has made it possible to retain or at least control such changes in their aforementioned characteristics. Thus as transistor gate length has been reduced, the gate dielectric layer""s thickness has also been reduced to at least partially compensate for the change in device output. Similarly, as the size of capacitor structures has been reduced, materials such as hemispherically grained polysilicon (HSG) have been employed to increase the effective surface area of such structures and compensate, at least in part, for such size reductions.
However, it appears likely that as scaling continues such exemplary compensation techniques may not be possible. Silicon dioxide (SiO2), with a dielectric constant of about 3.9, remains the most common material employed for gate dielectric layers. To maintain transistor output at an acceptable level, a transistor having a gate length of 0.1 micron will require an ultra-thin SiO2 layer with a thickness of about 2 nanometers (nm). Ultra-thin being defined herein as a thickness of about 5 nm or less.
The forming and use of such ultra-thin SiO2 layers is problematic for a variety of reasons since such layers consist of only a few layers of molecules. Thus only one additional or missing layer of molecules can have a dramatic effect on device performance; for example where a desired layer is four SiO2 molecules thick, a change of one molecule will change a characteristic such as the layer""s capacitance by as much as 25%. In addition, such thin layers typically exhibit high current leakage, for example due to band to band or Fowler-Nordheim tunneling. Such layers are also more susceptible to dopant penetration or diffusion through the layer thus changing the characteristics of an adjacent layer or region; for example boron diffusion from the gate electrode into the channel region alters channel characteristics.
One method of reducing these problems is the use of a thicker layer of an alternative dielectric material such as a metal oxide having a higher dielectric constant than that of SiO2. For the purpose of illustration, a metal oxide gate dielectric having an appropriately high dielectric constant can be formed with a thickness several times that of a SiO2 layer while having the performance characteristics of the thinner SiO2 layer. Thus the thicker metal oxide layer is said to have the equivalent oxide thickness (EOT) of the thinner layer. Alternate metal oxide materials such as titanium oxide (TiO2), aluminum oxide (Al2O3), tantalum oxide (Ta2O5) and others have therefore received attention as replacements for SiO2. However, such alternate materials must exhibit, in addition to a high dielectric constant (greater than that of SiO2), a large band-gap with a favorable band alignment, low interface state density, good thermal stability and the ability to be formed in a manner consistent with known semiconductor process methods at reasonable cost and yield. Unfortunately, many candidate metal oxide materials having an appropriately high dielectric constant, cannot meet these additional requirements. Thus it would desirable provide alternate dielectric materials and methods of forming such materials that are appropriate as a replacement for ultra-thin SiO2 layers.
Embodiments in accordance with the present invention provide dielectric materials, methods of forming such dielectric materials, and semiconductor devices that employ such dielectric materials. Such embodiments provide for the forming of a first metal-containing dielectric layer over a silicon-containing surface of a substrate and the forming of a second metal-containing dielectric layer on the first layer. In embodiments of the present invention, the first and second metal-containing dielectric layers encompass elements selected from Group IVB and Group IIIB of the Periodic Table of Elements, respectively.
In some embodiments, a silicon dioxide layer is first formed and a first metal-containing layer is formed overlying such silicon dioxide layer. Advantageously, the metal of the first layer encompasses an element that can combine with the oxygen of the silicon dioxide to form a metal oxide material of the first metal-containing dielectric layer and chemically reduce the silicon dioxide to silicon.
In some embodiments in accordance with the present invention, one or more metal-containing layers are exposed to an atmosphere that encompasses oxygen while heating the exposed layers to a temperature effective to transform such metal-containing layers to metal-containing dielectric oxide layers. In some embodiments, such exposing encompasses ion bombardment of the metal-containing layers, and in some embodiments such exposing encompasses providing oxygen radicals to the metal-containing layers.
Embodiments in accordance with the present invention provide for forming the first and second metal-containing dielectric layers having a wide ratio of relative thicknesses, for example from a ratio of about 1:5 to about 5:1 or greater. In embodiments of the present invention, such forming can be provided by physical vapor deposition (PVD) where a metal-containing layer of each formed layer is first deposited having a thickness of about 10 nanometers (nm) or less. In embodiments of the present invention, such PVD methods include electron beam evaporation techniques or other methods for forming high purity material layers by PVD. For example in some embodiments, radio frequency or microwave energy is employed for heating rather than an electron beam.
Embodiments of the present invention can encompass semiconductor devices such as MOS transistors, capacitors and the like. Such devices are formed using metal-containing dielectric layers in accordance with the present invention.
Some embodiments encompass integrated circuits such as dynamic and static random access memories (DRAMs and SRAMs) which include transistors, capacitors and the like that are formed employing metal-containing dielectric layers in accordance with the present invention, where such layers are also formed by methods in accordance with the present invention. In some embodiments of the present invention, semiconductor devices encompassing a metal-containing dielectric layer have an equivalent oxide thickness (EOT) of 2 nm to 5 nm. Advantageously, metal-containing dielectric layers in accordance with the present invention having an EOT of less than 2 nm are also possible.